Chemically strengthened glasses are glasses that have undergone a chemical modification to improve at least one strength-related characteristic, such as hardness, resistance to fracture, etc. Chemically strengthened glasses have found particular use as cover glasses for display-based electronic devices, especially hand-held devices such as smart phones and tablets.
In one method, the chemical strengthening is achieved by an ion-exchange process whereby ions in the glass matrix are replaced by externally introduced ions, e.g., from a molten bath. The strengthening generally occurs when the replacement ions are larger than the native ions (e.g., Na+ ions replaced by K+ ions). The ion-exchange process gives rise to a refractive index profile that extends from the glass surface into the glass matrix. The refractive index profile has a depth-of-layer or DOL that defines a size, thickness or “deepness” of the ion-diffusion layer as measured relative to the glass surface. The refractive index profile also defines a number of stress-related characteristics, including a stress profile, a surface stress, center tension, birefringence, etc. The refractive index profile defines an optical waveguide when the profile meets certain criteria.
Recently, chemically strengthened glasses with a very large DOL (and more particularly, a large depth of compression) have been shown to have superior resistance to fracture upon face drop on a hard rough surface. Glasses that contain lithium (“Li-containing glasses”) can allow for fast ion exchange (e.g., Li+ exchange with Na+ or K+) to obtain a large DOL. Substantially parabolic stress profiles are easily obtained in Li-containing glasses, where the ion-exchange concentration profile of Na+ connects in the central plane of the substrate, shrinking the traditional central zone of the depth-invariant center tension to zero or negligible thickness. The associated stress profiles have a predictable and large depth of compression, e.g., on the order of 20% of the sample thickness, and this depth of compression is quite robust with respect to variations in the fabrication conditions.
A stress profile of particular commercial importance is a near-parabolic (substantially parabolic) profile that has a “spike” near the surface. The transition between the parabolic portion of the profile and the spike has a knee shape. The spike is particularly helpful in preventing fracture when the glass is subjected to force on its edge (e.g., a dropped smart phone) or when the glass experiences significant bending. The spike can be achieved in Li-containing glasses by ion exchange in a bath containing KNO3. It is often preferred that the spike be obtained in a bath having a mixture of KNO3 and NaNO3 so that Na+ ions are also exchanged. The Na+ ions diffuse faster than K+ ions and thus diffuse at least an order of magnitude deeper than the K+ ions. Consequently, the deeper portion of the profile is formed mainly by Na+ ions and the shallow portion of the profile is formed mainly by K+ ions.
In order for chemically strengthened Li-containing glasses to be commercially viable as cover glasses and for other applications, their quality during manufacturing must be controlled to certain specifications. This quality control depends in large part on the ability to control the ion-exchange process during manufacturing, which requires the ability to quickly and non-destructively measure the refractive index (or stress) profiles, and particular the stress at the knee portion, called the “knee stress.”
Unfortunately, the quality control for glasses with spike stress profiles is wanting due to the inability to adequately characterize the profiles in a non-destructive manner. This inability has made manufacturing of chemically strengthened Li-containing glasses difficult and has slowed the adoption of chemically strengthened Li-containing glasses in the market.